Optical maser

ABSTRACT

1,015,154. Optical masers. WESTERN ELECTRIC CO. Inc. Oct. 4, 1962 [Oct. 16, 1961], No. 37498/62. Heading H3B. The resonant cavity of an optical maser is bounded by two parallel reflecting surfaces 21 and 24 between which is placed at least one partially transmitting reflector 22, 23 to divide the cavity into a number of gaps at least one of which contains a negative temperature material. The effect of the additional reflecting surfaces is to improve the made discrimination and it is stated that for optimum results the laser of Fig. 2 should have the reflectivity of the end mirrors 21 and 24 at least as great as that of mirrors 22 and 23. It is also stated that the length of the gaps which do not contain negative temperature material should be (2#v)&lt;SP&gt;-1&lt;/SP&gt; where #v is the half width of the fluorescent emission line of the active medium.

May 26, 1954 P. P. KlsLluK ETAL 3,134,837

OPTICAL MASER Filed Oct. 16, 1961 4 www P. P. /f/SL/UK /Nl/E/VTORS United States Patent fice' 3,134,837 Patented May 26, 17964 3,134,337 OPTICAL MASER Paul P. Kislink, Morristown, and David A. Kleinman,

Plainfield, NJ., assignors to Bell Teiephone Laboratories, Incorporated, N ew York, NSY., a corporation of New York Filed Oct. 16, 1961, Ser. No. 145,987

" v3: Claims. (Cl. 8 8-1) This'invention relates to optical masers and, more particularly, to cavityresonators for use in such devices.

The recent invention of the'optical maser has made possible thegeneration and amplification of coherent electromagnetic waves in the optical frequency range. This range is generally considered to extend from the farthest infrared portion of the spectrum throughthe ultraviolet. Due to the extremely high frequencies associated with wave energy in this range'fthe coherent waves produced optical inaser devices are capable of transmitting enormous quantities of information. Thus, the resulting extension of the usable `portion of the electromagnetic spectrum has greatly increased the number of frequency channels available for communication and other uses.

As developed for use at microwave frequencies, masers typically comprise a negativev ltemperature medium which is contained in a cavity resonator having 'a single resonant mode near the frequency at which stimulated emission is to be produced. The design of such cavity resonators for microwaves is a relatively simple matter, typical structures having dimensions "onv the order" of a single wavelength at the chosen frequency. The application of this design approach to optical masers is impractical, however, due to the extremely small wavelengths involved. It has been necessary, therefore, to design optical cavity resonators having dimensions which may be thousands of times lrger'than'the wavelength of signals at"the operating frequency.V

One such structure which has been employed success- Vfully for the specified purpose is the Fabry-Perot interferometer comprising two plane parallel reflective surfaces separated by af gap of"convenient "lengthl The active medi'mlof the mseris located in the gap between vthe reflective surfaces, at least one of which is partially transmissive 'to" permit "coupling the device to an external Vutilization circuit. An optical maser of this type is described in US, Patent 2,922,222 to Schawlow and Townes.

Optical cavity resonators, being of necessity much larger than the wavelengths employed therewith, are inherently multimode devices. A mathematical analysis of the mode lsystem in a Fabry-Perot resonator having reflecting end surfacesmay be found in an article by Fox and Li in the Be11`sjytmrhnica1 Journal, vol. 40, page' 45.3. 'Fox and Ili'hve shown thatthe' resonator may be characterized bye -Ilunlberpf axialV andfoff-axial modeshaving very lowlsfsesl In a"device"having'the :configuration analyzed in the abovernrentioned article, losses are theV same in all axial modes. Thos, no axial mode is preferred and sellral may be excited simultaneously.

yThe presence' of many modes in a maser adapted for communication purposes, however, is disadvantageous. For example, significantly more power is required by a multimode than a single mode maser device to produce the desiredwell defined output line which stands out :learlyffr'iinnl tliebackgr'ound emission. Furthermore, the

Aexcitationof many modes an'adverse effect Yon the sta on the maser, an important consideration in com# Ymunrcations systems.

v"It is n'object of this invention to provide an optical maserc'avity resonator: having 'ai inode' lsystem which includes a relatively few preferred modes among a plurality @t nos@ andes'- It is also an object of this invention to increase `the losses of certain modes in the cavity resonator rof an optical maser, relative to other preferred modes therein.

These and other'objects of the invention are achieved Y in one specific illustrative embodiment thereof comprising two`spaced flat parallel reflective surfaces `defining therends of an optical cavity resonator, and a third flat In a second embodiment illustrative of the principles of the' invention four axially spaced flat'parallelzrefiective surfaces define an'optical cavity resonator of three gaps. In this embodimentthe negative temperature medium 1s advantageously contained in the center gap.

It is a feature of the invention that the refieetivesurface or surfaces interior to the overall ycavity defined by the two end reflective surfaces are partially transmissive to light at the operating frequency of the maser.

" It is also `a feature of the invention .that at least one of the two end reflective surfaces ,definingthe' overall cavity is'partially transmissive It may be desirable in some instances that both end reflective" surfaces be partially tfransmissive. Such possibility is also in accordance with the invention. i

It is a further feature of the invention that at least one end reflective surface of' the cavity, which defines one end ofa gap therein, has a reflectivity at least .as'great as that of the interior reflective surfacedefining the other end of the ,samevgap v The above-mentioned and other objects and features rof the 4invention will be better understood from, the following rnore detailed discussion taken in conjunction with the accompanying drawing, in which:

FIGA depicts a first illustrative optical maser 4'embodyiing thePinciples'ofthe invention; v

FIG. 2 depicts a second optical maser embodyingv the invention; FIG.' 3 is a schematic diagram illustrating the invention; l' '4 FIG. 4a illustrates the mode spectrum ofy a two-surface optical cavity resonator; and A f' FIG. 4 b`illustrates the mode spectrum of a four-surface optical cavity resonator.

The optical maser 10 shown in FIG. 1 comprises an optical cavity resonator rformed by three axially spaced, fiat parallel reflective surfaces 11, 12 and 1'3. An active maser medium in the form of a rod 15 is ydisposed within the cavity in the gap between reflective surfaces 12 and 13, While the gap between surfaces 11 and 12 contains-a substantially transparent inert medium such as air Vor a vacuum. The surfaces 12 and 13 may be formed lby evaporating thin lfilms of, silverV or other reflcctivevsub; stance directly onto flat ends of the rod 15. A lamp 16, connectedto apower source, not shown, is arranged about the rod`15f for' supplying"pump. wave energy thereto. Maser action initiated when the pump power produces a population inversion in the energy level system ofrodlSr.4 Optical masers of the type hitherto known, employ# ing the Fabry-Perot interferomete'ras a resonant cavity, are characterized by a number ofA resonant'inodes some of which it is desired to suppress. Such modes tend to degrade the performance ofthe maser and aretroublesome whenever the fluorescent emission of the device covers a frequency band Wider Vthanabout (2nd) T1. numbers, where n is the refractive 'index of the active medium filling the cavity andl i .is kthe distance between the reiiective ends thereof. YIt is believed that these modes are the cause of several types of fine structure which has been observed in the output beams of optical masers whose operation has been reported in the literature. Decreasing d is not a satisfactory way of avoiding the unwanted modes from the output, primarily because this would also decrease the amount of active material in the maser and, hence, decrease the gain of the device.

Mode suppression is achieved in tthe optical maser by adding an additional reflective surface to the priorly known Fabray-Perot resonator structure. Thus, in the device shown in FIG. 1, the reflective surfaces 12 and 13 are partially transmissive to light at the operating frequency, while the surface V11 is totally reflective. The addition of another reflective surface to the resonator structure effectively discriminates against the undesirable modes of the basic two-surface resonator by increasing their losses relative to the losses in other modes. In general', the desired result is achieved when the length of the empty gap between surfaces 11 and 12 is less than that of the gap between surfaces 12 and 13. Optimum discrimination is obtained in the optical maser 10 when the empty gap is about equal to (2h11)-1 where Av is the half-width of the fluorescent emission line.

The invention is not limited to the embodiment shown in FIG. l but may also be applied, for example, to an optical maser of the type illustrated in FIG. 2. The maser depicted therein comprises spaced parallel rellective surfaces 21, 22, 23 and 24. The overall optical cavity resonator is defined by end reflective surfaces 2.1 and 24, at least one of which is partially transmissive to light at the operating wavelength of the device. The reflective surfaces 22 and 23 are partially transmissive and, in conjunction with surfaces 21 and 24, define three interior'gaps of the resonator. The active maser medium, in the form of a rod 15, is located in the center gap between surfaces 22 and 23. Y

In accordance with one feature of the invention each reflective end surface of the optical cavity which also defines one end of an empty gap therein, has a reflectivity which is at least as great as that of the interior reflective surface forming the other end of the same empty gap. Thus, in the optical maser 10 shown in FIG. 1 the surface 11 'is totally reflective and so has a reflectivity greater than that of the interior surface 12, which is partially transmissive. It is to be noted, however, that the mode discriminating characteristics of the invention are not enhanced by making the surface 11 more reflective than the surface 12. Optimum conditions for mode discrimination require that the surface 11 be at least as reflective as the surface 12, but other considerations will determine whether or not its reflectivity exceeds that of surface 12.

In the optical maser 20 depicted in FIG. 2, reflective surfaces 21 and 24 define the overall cavity. Empty gaps are defined by surfaces 21 and 22, and by surfaces 23 and 24. The surface 21 is totally reflective while surface 22 is partially transmissive to light of the operating frequency. Thus, the reflectivity of the surface 21 is at least equal to that of the surface 22, as required by the invention. At the other end of the maser 20, both reflective surfaces 23 and 24 are partially transmissive. In accordance with the invention, however, the reflectivity of the end surface 24, which defines one end of an empty gap, is at least as great as the reflectivity of the surface 23.

The properties of the invention may be understood by referring to FIG. 3, which is a two-dimensional schematic representation of an optical maser having four reilective surfaces. The active medium, indicated by the shaded area at -azca is characterized by a real dielectric constant e 1 and a real conductivity a. For Iz[ a it is assumed that e=l and 0:0. Reflective surfaces are placed at z\=ia, ib and have retlectivities r,L and rb respectively.

Ve-I-l and we may define Ttanh f and write It is then possible to consider arbitrary reilectivities at z: ia, ib by suitable choices for T and \/e in the range 0 tol.

Furthermore, allow the angular frequency w of the light waves in the cavity to be real, and a to assume an appropriate negative Value. Now if the dimensions are chosen so that when m and n are positive integers, then it can be shown by considering the electromagnetic ileld in the cavity that for the preferred mode having the lowest loss where X=tanh (21rra/c\/e) is a measure of the loss. For the modes having the largest losses Let the quantity R X X min.

be called the discrimination ratio. Then Rm.,x isV equal to e or to whichever is smaller. It follows that, for optimum discrimination among the modes in the cavity rb should be equal to or greater than ra.

Consider a specific example \/F=1O and T=.02. The corresponding reflectivities are ra=.82 and rb=.96. The loss of the preferred mode is in which m/n=1/s,

Xmin: -002 while In FIG. 4 the mode spectrum of the arrangement depicted in FIG. 3 is compared with that of an arrangement having no reflective surfaces at ib. The loss in the two-surface case (FIG. 4a) is identical for all modes:

t. Y The heights of the spectrum lines in FIG. 4 are proportional to two-surface arrangement. However, the additional modes are among the lossy ones and do not interfere with the desired result. The periodicity in the mode spectrum shown in FIG. 4b is the result of choosing an integer. Periodicity is destroyed by making nonintegral, but there is still a preferred mode with minimumrloss. The greatest advantage in discrimination against unwanted modes is obtained by setting Where Av is the half-Width of the lluorescent emission line of the active medium.

Although the invention has been described with particular reference to specific embodiments, many modifications and variations are possible and may be made by those skilled in the art without departing from its scope and spirit.

What is claimed is:

1. An optical maser comprising an elongated optical cavity resonator having flat parallel reflective end members, at least one flat reflective interior member within said resonator spaced axially from said end members and parallel thereto, said interior member and at least one of said end members being partially transmissive to light wave energy at the operating frequency, at least one end member of said resonator having a reflectivity substantial-ly as great as that of the facing interior reflective member of said resonator, said end and interior members defining a plurality of gaps in said cavity resonator, at least one gap in said resonator having an optical length substantially equal to the reciprocal of the Width of the fluorescent emission line in wave numbers, and active maser medium disposed in at least one of said gaps, and means for applying pump wave energy to said medium for producing a population inversion therein.

2. An optical maser comprising an elongated optical cavity resonator having flat parallel reflective end members, irst and second flat reflective interior members within said resonator spaced axially from said end members and parallel thereto, said interior members and at least one of said end members being partially transmissive to light wave energy at the operating frequency, each end reflective members having a reflectivity substantially as great as that of the nearest interior reflective member, said end and interior members defining three gaps in said resonator, an active maser medium disposed in the center gap of said resonator, the two empty gaps having lengths substantially equal to the reciprocal of the Width of the iiuorescent emission line in wave numbers, and means for applying pump Wave energy to said medium for producing a population inversion therein.

3. An optical maser comprising an elongated optical cavity resonator having flat parallel end reflective members, a single flat reflective interior member Within said resonator spaced axially from said end members and parallel thereto, said interior member and at least one of said end members being partially transmissive to light Wave energy at the operating frequency, the end member of said resonator which denes one end of an empty gap therein having a reilectivity substantially as great as that of said interior reective member, said end and interior members dening two gaps in said resonator, an active maser medium disposed in one of said gaps, the empty gap in said resonator having a length substantially equal to the reciprocal of the width of the fluorescent emission line in Wave numbers, and means for applying pump wave energy to said medium for producing a population inversion therein.

References Cited in the tile of this patent Jenkins et al.: Fundamentals of Optics, 2nd ed. (1950), pages 269, 272-3.

Morgan: Introduction to Geometrical and Physical Optics (1953), page 232.

Boyd et al.: Bell System Technical Journal, vol. 40, No. 2, (March 1961), pages 491, 494, 504 and 505.

Fox et al.: Bell System Technical Journal, vol. 40, No. 2, (March 1961), pages 481 to 483. 

1.AN OPTICAL MASER COMPRISING AN ELONGATED OPTICAL CAVITY RESONATOR HAVING FLAT PARALLEL REFLECTIVE END MEMBERS, AT LEAST ONE FLAT REFLECTIVE INTERIOR MEMBER WITHIN SAID RESONATOR SPACED AXIALLY FROM SAID END MEMBERS AND PARALLEL THERETO, SAID INTERIOR MEMBER AND AT LEAST ONE OF SAID END MEMBERS BEING PARTIALLY TRANSMISSIVE TO LIGHT WAVE ENERGY AT THE OPERATING FREQUENCY, AT LEAST ONE END MEMBER OF SAID RESONATOR HAVING A REFLECTIVITY SUBSTANTIALLY AS GREAT AS THAT OF THE FACING INTERIOR REFLECTIVE MEMBER OF SAID RESONATOR, SAID END AND INTERIOR MEMBERS DEFINING A PLURALITY OF GAPS IN SAID CAVITY RESONATOR, AT LEAST ONE GAP IN SAID RESONATOR HAVING AN OPTICAL LENGTH 